i am not sure about this answer:
float x = 1.0F;
float y = 1.0F;
Bitmap bitmap = new Bitmap(PathToYourImage/StreamOfYourImage);
Graphics g = Graphics.FromImage(bitmap);
g.ScaleTransform(x,y);
there you have to change x and y which is a scalarfactor
lets say your image width is 600 and you want to have 800 so:
float x = (float) 800/600;
=> float x/y = (float) <new Width/Height> / <old Width/Height>
hope that helps
i am not sure if you can also achive that effect by the Bitmap constructor, because there you can tell the image width and height, but my method also is a good training for image editing :)